Various chemically reactive components, such as the components of two-part synthetic-resin systems (e.g. epoxy and urethane systems) must be combined before the reaction mixture is introduced into a mold or other shaping cavity in an intimate and uniform manner.
To this end, it has been proposed to provide mixing heads which form chambers into which the respective streams of the reactive components (resin and curing agent in the case of epoxies and polyol and diisocyanate in the case of urethane) open so as to permit mixing of the two components before or as the mixture is discharged axially from the mixing chamber into the cavity.
Such systems eliminate the need for agitators, stirrers and like movable mixing elements which are conventionally used in a mixing chamber to bring about an intimate combination of the two components.
In this connection, mention can be made of the following publications which deal with mixing heads of the type with which the present invention is concerned.
German Pat. No. 2,007,935 which corresponds to U.S. Pat. No. 3,706,515; PA1 German utility model (Gebrauchsmuster) No. 7,310,963; PA1 German Pat. No. 2,117,533 (which corresponds to U.S. Pat. No. 3,799,199); and PA1 German Pat. No. 1,282,522 (which corresponds to U.S. Pat. No. 3,263,928).
Reference may also be had to German Pat. No. 1,225,374, to German published application (Auslegeschrift) No. 1,255,297, German utility model (Gebrauchsmuster) No. 1,814,838, French Pat. No. 1,376,485, British Pat. No. 863,843, U.S. Pat. No. 3,385,671, and U.S. Pat. No. 2,868,518 which were cited during the examination of German Pat. No. 2,007,935 mentioned above.
In German Pat. No. 2,007,935 and U.S. Pat. No. 3,706,515, a mixing head of the aforedescribed type is provided for synthetic-resin components which has a mixing chamber with lateral inlet ports for these components and an axial discharge opening.
A control plunger or piston is shiftable in this mixing chamber and is formed with recycling passages which, in one position of the plunger, connect each inlet port of a respective component to a return passage for this component and to a discharge port whereby the components circulate through the mixing head without contacting each other. In the open position of the plunger, the inlet ports communicate with the chamber so that the streams of the two components can mix and discharge through the axial outlet. In these systems, moreover, the plunger, in being displaced between the mixing position and the recycling position can discharge the residues of the mixed product from the chamber.
The recirculating or nonmixing phase assures continuous movement of components which might otherwise tend to congeal or gel.
When such mixing heads are operated at high pressure, it is of concern that in the nonmixing phase, i.e. when the synthetic-resin components supplied to the mixing head are recycled to the reservoir, the return should involve absolutely no mixing of the two components with one another. Obviously should mixing occur, hardening, congealing or gelling of the resulting mixture will also occur to block the mixing head or render it inoperative at least until it is cleared. The resulting synthetic-resin product also constitutes an impurity in the mixture itself when mixing is to resume and the mixture is to be discharged into the mold.
At high pressures, even with utmost care in the machining of the earlier mixing heads, it has been found that some seepage has occurred between the two components so that an undesired mixture is formed in the nonmixing or recirculating phase.
Problems are also encountered when one or both of the components contains a filler, which may be present in significant proportion to conserve the relatively expensive synthetic-resin components themselves. Typical fillers are glass fibers, limestone, and the like. In such cases, it is found to be advantageous to introduce the filled component into the mixing chamber through a plurality of inlet ports (e.g. three) as described for example in German utility model No. 73 10 963. Here again, a sealing problem arises because the mixing chamber, whose diameter is usually only about 10 mm, cannot accommodate more than two inlet ports at the same level or in the same plane without permitting some leakage around the axially movable control body and between the two liquids. Again, in these situations, the two components mix where they meet each other to form the synthetic-resin material in a detrimental manner.
Attempts have been made to solve the problem by forming the control piston or plunger with sealing grooves between the bypass or return flow passages (see German Pat. No. 2,117,533 and U.S. Pat. No. 3,799,199); the leaking synthetic-resin components, entering these grooves, mix together and form in situ a sealing mass or strand which blocks further interaction between the two components in the nonmixing phase.
While this latter technique has worked out well in practice, in recent years greater demands have been placed upon such mixing heads which must handle increasingly greater flow rates and pressures, to the point where earlier sealing systems have not been found to be wholly satisfactory.